Method of reacting cellulose paper and specific non-ionic latices containing hydrogen and hydroxy groups in the polymer chain with polyisocyanate adducts



United States Patent M METHQD 0F REACTING CELLULUSE PAPER AND SPECIFICNON-HUNMI LATICES CQNTAINING HYDROGEN AND HYDRGXY GROUPS IN THE PGLYMERCHAHN WITH PULYISOCYANATE ADDIUCTS Emile E. Hahih, Boston, and MelvinNimoy, Hyde Park, Mass, assiguors to W. R. Grace dz (10., Cambridge,Mass, a corporation of Connecticut No Drawing. Uriginal application.lune 2, 1959, Ser. No. 317,488. Divided and this application Mar. 31,196i, Ser. No. 102,085

9 Claims. (Cl. 8-4156) This application is a divisional application ofcopending application Serial No. 817,488, filed on June 2, 1959, and nowabandoned.

This invention relates to a method of treating paper to improve thephysical properties of the same.

It is common knowledge that the physical properties of an ordinary sheetof paper are drastically impaired when the sheet is wet. It is alsoknown that the physical properties of the same sheet would bephenomenally increased even when the sheet is wet, if the paper isimpregnated with a polymeric latex, dried, and then immersed into anorganic solution containing an isocyanate. In theory, it is thought thatthe isocyanate, which is used, functions to cross-link the elastomericpolymer to the paper by means of urethane bridges thereby mechanicallybonding the fibers of the paper to the polymer. It is also thought thatthis cross-linking decreases the hydrophilic nature of the fibers of thepaper so that they absorb less water. In any case, it has been foundthat a sheet of paper, which has been treated in this manner, exhibitsan enhanced degree of tensile strength and abrasion resistance eventhough the sheet may be wet.

The basic difliculty with this process is that it was found necessary,in each and every case, to dissolve the isocyanate in a non-reactivesolvent in order to use the isocyanate in the treatment of paper. Ifthis is not done, the isocyanate, when used for treatment, reactspredominantly at a fairly rapid rate, with the active hydrogens presentin the latex system, forming insoluble di-urea compounds. When thishappens, a substantial quantity of the isocyanate is consumed andthereby becomes unavailable to cross-link the elastomeric polymer andthe paper.

The use of solvents has been found to be a serious hindrance to thecommercialization of this process for reasons of economics and safety.This treatment is expensive because the solvent absorptivity ofimpregnated type paper may be as high as 100% based on the weight of thepaper even after squeezing out the excess with pressured rolls. Needlessto say, this treatment could be quite expensive in view of the amount ofsolvent that is absorbed unless care is taken to recover the absorbedsolvent. A suitable solvent recovery system would be expensive toconstruct and to operate especially in view of the large quantities ofpaper that wolud be involved if this method of treatment was used with ahigh speed paper machine. A further deterrent to the use of thistreatment is the fire hazard associated with the use of large amounts ofsolvent. Another deterrent is that this treatment is essentially atwo-stage process in which the latex treated paper must be removed toanother machine and treated with the isocyanate solution in a separate3,238,0W Patented Mar. ll, 1966 operation. It is evident that the use ofsuch a treatment be deterred except where absolutely necessary.

We have developed a method whereby paper may be impregnated with latexand treated with isocyanate without the use of solvent and without thenecessity of a twostep process. In this method the isocyanate isstabilized to the presence of active hydrogens by reacting theisocyanate with an agent which renders the isocyanate group inactive atordinary temperatures but which permits the isocyanate to be regeneratedand thereby become reactive when heated to a temperature above about C.Since we are able to add the isocyanate, in unreactive form, to thelatex, we have been able to impregnate the paper with the polymericlatex, as well as the isocyanate, in a single operation. This treatmentimparts the same properties to paper as does a treatment with latex,followed by a subsequent treatment with an isocyanate dissolved in anorganic solvent. The difliculties which industry has heretoforeencountered with the utilization of a solvent solution are eliminated.

In general, our process comprises treating a sheet of paper with a latexcontaining a polyfunctional isocyanate in unreactive form, and thenregenerating the isocyanate in active form, while in contact with thepaper. For claritys sake, all polyfunctional isocyanates are hereinreferred to as isocyanate.

More particularly, our process comprises impregnating a sheet of paperwith a latex containing an isocyanate in unreactive form, drying thepaper at a temperature below 140 C., and heating the impregnated paperto a temperature above 140 C. to regenerate the isocyanate in a formwhich is reactive to both the cellulosic fibers in the paper and theelastomeric chain in the latex.

In the practice of the invention, an isocyanate in unreactive form isincorporated into the aqueous latex most generally by means of anemulsion. The isocyanate is one in which the reactive groups of theisocyanate molecule have been rendered inactive at ordinary temperaturesby the formation of a chemical complex, which complex when heated,decomposes to regenerate the isocyanate molecule in the freely reactiveform. Paper is thus impregnated with latex containing this isocyanate inunreactive form. Subsequently, this treated sheet is dried and then isheated and the isocyanate groups contained therein are therebyregenerated. It is known that cmpounds which contain free isocyanategroups are, most generally, very reactive compounds and that the freeisocyanate groups of these types of compounds will react withpractically any active hydrogen compound, i.e. a compound containing ahydrogen which may normally be replaced with sodium. The free isocyanategroups even react with the hydrogen and hydroxyl groups present in thecellulosic chain of the paper and in the elastomeric chain of thepolymer. As a result of such a reaction the cellulosic fibers appear tobe interconnected to each other by means of elastorneric chains whichtie the fibers into a network wherein the elastomeric chains are bondedto the cellulosic fibers by means of urethane bonds.

ISOCYANATES There are many organic molecules containing isocyanategroups which will react with the polymer in the latex and with thecellulose fibers in the paper. In general, any of the polyisocyanates inthe following classes, due to their chemical nature are reactive Withgroups containing e: an active hydrogen. Suitable isocyanates includethe aliphatic diisocyanates such as ethylene, trimethylene,tetramethylene, pentamethylene, hexamethylene, propylene- 1,2butylene-1,2, butylene-2,3, butylene-1,3, ethylidene, and butylidenediisocyanate; the aromatic diisocyanates such as m-phenylene,p-phenylene, 4,4-diphenyl, 1,5- naphthalene, and 1,4-naphthalenediisocyanate; the cycloalkylene diisocyanates such ascyclopentylene-l,3, cyclohexylene-l,4, and cyclohexylene-1,2; thealiphatic aromatic diisocyanates such as 4,4-diphenylene methane, 2,4-tolylene, 4,4-tolidene, and 1,4-xylylene diisocyanate; the nuclearsubstituted aromatic isocyanates such as dianisidene, 4,4'-diphenylether and chloro-diphenylene diisocyanate; the triisocyanates such as4,4,4"-triisocyanato triphenyl methane, 1,3,5-triisocyanato benzene, and2,4,6- triisocyanato toluene; and tetraisocyanates such as 4,4-dimethyl-diphenyl methane and 2,2,5,5-tetraisocyanate.

INHIBITING AGENTS As pointed out above in the practice of our inventionthe active isocyanate group is reacted with a chemical agent whichrenders the isocyanate group inactive at ordinary temperatures, butwhich degenerates thereby regenerating the isocyanate, in active form,upon heating to temperatures above about 140 C. There are many chemicalreagents which will accomplish this result. These include aceto-aceticester, diethylmalonate; mercaptans such as 2-mercapto-benzothiazole,lactams such as epsilon-caprolactams, delta-valerolactam,gamma-butyrolactam, and beta-propiolactam; imides such as succinimideand phthalimide; tertiary alcohols such as ter-amyl, tertiary butyl,dimethyl ethenyl carbinol, dimethyl phenyl carbinol, methyl diphenylcarbinol, triphenyl carbinol, 1- nitro tertiary butyl carbinol,l-chlorotertiary butyl carbinol, and triphenyl silinol; secondaryaromatic amines such as diphenyl-amine; diaryl compounds such asdiphenylamine, o-ditolyl amine, m-ditolyl amine, p-ditolyl amine,N-phenyl toluidine, N-phenyl xylidene, xylidene, carbazole, phenyl alphanaphthylamine, and phenyl beta naphthylamine; dimers of aromaticmono-isocyanates; bisulfite addition products, phenols and cresols.

POLYMERIC LATEX The polymeric latices used in this treatment must benon-ionic in nature. It has been found that the stability of the blockedisocyanate to premature cleavage is excellent in non-ionic latices andpoor in either anionic or cationic latices. The polymeric systems, whichare used, however, must contain labile hydrogen or hydroxy groups withwhich the isocyanate, when regenerated in active form, may react. Thepolymeric latices which are preferably used are polymers which can bemade in non-ionic systems. Examples of non-ionic latices are thosecontaining polyvinyl acetate, copolymers of vinyl acetate, andcopolymers of butadiene-acrylonitrile such as that commerciallyavailable under the trade name of Hycar 1872.

ACTIVATOR The reactivity of the polymeric system may be further enhancedby increasing the number of labile hydrogen or hydroxy groups present inthe system. This may be accomplished, in the case of polyvinyl acetate,by a partial hydrolysis of the system. It may also be accomplished, inthe case of both polymers and copolymers of vinyl acetate, byintroducing reactive groups into the molecule during the polymerizationof the monomer, such as by incorporating hydroxy bearing emulsifyingagents into the reactive charge during the emulsion polymerization ofthe monomer. It has been noted that only about 50% of such hydroxybearing material has been recovered after the polymerization reactionhas been completed indicating that the hydroxy bearing material, whichhas not been recovered, had been incorporated into the polymer. Thereare a number of hydroxy bearing materials which may be used for thisgrafting technique, such as polyvinyl alcohol, hydroxy-ethyl celluloseand carboxymethyl cellulose.

EXTENDERS It has also been found that these polymeric latices may beeconomically extended with emulsions of polymerized petroleum resins,low molecular weight oils, or other extenders which will not filter outduring impregnation of paper. We have preferentially used emulsions ofthe polymerization types of petroleum resins such as those commerciallyavailable under the trade name of Piccopalc N-2.

PAPER The paper which is to be treated by this method, should besufficiently porous that the ingredients of the impregnating compositionwill not be filtered out by the paper during treatment. The minimumdegree of necessary porosity is controlled by the particle size of theparticular polymer present in the latex. Particle size, of course,varies depending not only on the method of producing the polymer butalso on the specific monomers used in the process. Since the method ofproducing suitable porous papers is an art itself which is well known tothe paper industry it is not considered necessary to consider thissubject in detail.

DISPERSION OF THE ISOCYANATE IN THE LATEX The isocyanate material in theunreactive or blocked state is introduced into the latex in the form ofa dispersion or of an emulsion in water. A dispersion in water may beformed by ball milling the blocked isocyanates in the presence of waterand a dispersion agent. We prefer, however, to form an emulsion of theblocked isocyanate since we have found that when added as an emulsionthe isocyanate is more intimately mixed with the latex. A rathersatisfactory emulsion may be formed by dissolving the blocked isocyanatein ethyl acetate, and then adding the solution to a warm aqueoussolution containing sodium alkyl naphthalene sulfate, alkyl aryl sodiumsulfonate, and ammonium caseinate. In the emulsion the ammoniumcaseinate functions as a stabilizer for the emulsion, the sodiumnaphthalene sulfate functions as a wetting agent for the system, and thealkyl aryl sodium sulfonate functions as a dispersing agent for theorganic phase.

TREATMENT OF PAPER The paper may be impregnated with the impregnationcompound in a number of ways. These include beater impregnation, wet webimpregnation and dry web impregnation. For convenience the examples thatfollow were made by dry web impregnation by passing paper through a bathcontaining the latex solution in which the blocked isocyanate wasdispersed. After impregnation, the paper was further processed bypassing the same through a conventional press or calender in order tosqueeze out the excess impregnant. Subsequently, this paper was heatedat a low temperature to substantially dry it and then heated to atemperature between about 100 C. and 180 C.

p for a period up to 10 minutes to regenerate the isocyanate andcross-link the polymer and paper.

The following examples show how various blocked isocyanates aredispersed in a vinyl acetate copolymer latex and how these dispersionsare used to treat paper. All formulations given herein are on the drybase for all ingredients. The treated sheets were then conditioned bymaintaining them at a temperature of about F. and a relative humidity ofabout 50% for 14 to 16 hours. After conditioning, the treated sheetswere tested to determine their tensile strength.

Tensile strength-The treated sheets were tested on a Scott tensiletester, Model X-3. These measurements were made on /2" x 4" strips oftreated paper with a one inch gap between the jaws. Elongation was madeat the rate of 12" per minute and the tensile was reported as pounds(corrected to an equivalent 1" strip). Prior to the wet tensile test,the paper was soaked in water at 70 C. for 24 hours.

In Example I, a blocked isocyanate was emulsified and then dispersed ina vinyl acetate copolymer latex which was later used to treat paper.

Example I 50 parts by weight of the reaction product of toluene,2,4-diisocyanate, trimethylol propane and phenol were ground into acoarse powder and disolved in ethyl acetate. This solution was thenemulsified in another solution containing 3 parts by weight of ammoniumcaseinate, 5 parts by weight of sodium alkyl naphthalene sulfate, and0.5 part by weight of alkyl aryl sodium sulfonate dissolved in 23 partsby weight of warm water. The emulsion was then homogenized and stirreduntil the temperature of the emulsion was below about 110 F.

Various quantities of the emulsion were then added to an aqueous latexof non-ionic vinyl acetate copolymer which was the reaction product ofthe emulsion polymerizaiton of 77 parts by weight of vinyl acetate and23 parts by weight of dibutyl maleate. The resulting mixture was thenstirred until it was uniform.

8" x 10 sheets of impregnated base paper were then totally immersed intothis liquid mixture, drained, placed between sheets of blotting paperand then passed through a roller to simulate the squeeze rolls used incommercial practice. The sheets were dried, and then heated to atemperature of about 170 C. for 2 minutes to insure complete reactionbetween the paper and the isocyanate.

Table I sets forth the results obtained when the amount of blockedisocyanate in the system is varied. The papers were impregnated at alevel of about 20 parts by weight of polymer per 100 parts by weight ofpaper. It is clear that the wet tensile strength and wet elongation ofthe impregnated paper increase as the amount of blocked isocyanate inthe impregnant is increased. The increase in wet tensile is quite rapidas the amount of isocyanate is increased to 20 parts by weight. Theoptimum amount of isocyanate in the impre'gnant is between and parts byweight. The wet tensile strength of a sheet treated with such animpregnant would be 8 to 9 times that of a control sheet which was nottreated with isocyanate. Also, the effect on elongation closely followsthat of the tensile.

1 Expressed in parts by weight per 100 parts by weight of polymer. 2Expressed as pounds per inch. 3 Expressed as percent.

Example 11 10 parts by weight of the reaction product of toluene2,4-diisocyanate, trimethylol propane and phenol were emulsified as setforth in Example I. This emulsion was then added to 100 parts by weightof a mixture which contained varying ratios of a vinyl acetate copolymerlatex (same as that used in Example I) and a resinous materialcommercially available under the tradename of Piccopale N-2. Sheets ofimpregnated base paper were then treated, conditioned and tested asheretofore set forth.

The effects on the physical characteristics of a treated sheet wereobserved and listed in Table II. The level of total impregnant absorbedby the paper was 20 parts by 6 weight per 100 parts by weight of paper.It appears from this data that 60% to 70% extenders may be added to theimpregnant without severally affecting the tensile strength orelongation of the sheet.

TABLE II Tensile Strength 2 Latex 1 Extender 1 Elongation, 3

Dry Dry Wet 1 Expressed as parts by weight of impregnant. 2 Expressed aspounds per inch. 3 Expressed as percent.

Example 111 5 parts by weight of the reaction product of toluene2,4-diisocyanate, trimethylol propane, and phenol was emulsified as setforth in Example I. This emulsion was then added to 100 parts by weightof a polyvinyl acetate latex (same as that used in Example I). Sheets ofimpregnated paper were then treated at various levels of impregnation,conditioned and tested as heretofore set forth.

The results were then set forth in Table III. It appears that the wetand dry physical properties of a treated sheet are increased as thelevel of impregnation is raised.

TABLE III Tensile Strength 3 Elongation 4 Impregnation Amount Level 1Blocked Isocyanatc 2 Dry Wet Dry Wet v hundred parts by weight of paper.

1 Expressed as parts by weight of impregnant per parts by weight ofpaper.

Example IV Varying amounts of the reaction product of toluene2,4-diisocyanate, trimethylol propane, and phenol were dispersed in 100parts by weight of a vinyl acetate copolymer latex by means of a ballmill. The latex used was the same as that described in Example I. Sheetsof impregnation base paper were then treated with this dispersion,conditioned and tested as heretofore described.

In Table IV, the wet tensile strength of sheets treated by the emulsiontechnique (set forth in Example I) were compared with sheets treated bythe technique used in Example IV. The significant difference betweenthese two methods is the manner in which the blocked isocyanate isdispersed in the latex. These papers were impregnated at a level ofabout 22 parts by weight per It appears that no matter which techniqueis used the wet tensile strength of 21 treated sheet is greater thanthat of a control sheet which did not contain isocyanate. Also, themethod of dispersing the isocyanate in the latex appears to have asignificant effect on the Wet tensile strength of the sheet. The wettensile strength of a sheet treated according to procedure set forth inExample I is greater than a sheet treated by this procedure because theblocked isocyanate is more finely distributed in the latex when it is inemulsified form.

1 Expressed as parts by weight per 100 parts by weight of polymer. 1Expressed as pounds per inch width.

In Example V, the diphenylamine adduct of toluene 2,4-diisocyanate wasprepared, emulsified and then dispersed in a vinyl acetate copolymerlatex which was later used to treat paper.

Example V 17.4 grams toluene 2,4-diisocyanate were dissolved in 1000 ml.of toluene, 34 grams of diphenylamine were then added to this solutionand it was stirred for 2 hours and then allowed to stand for 12 hours.The toluene was then distilled from the liquid at which time the liquidwas cooled to 20 C. and filtered to separate the precipitate.

4 parts by weight of the precipitate (which was the diphenyl amineadduct of toluene 2,4-diisocyanate) was ground into a coarse powder anddissolved in 6 parts by weight of ethyl acetate. This solution was thendispersed in another solution containing 2 parts by weight of ammoniumcaseinate, 0.5 part by weight of sodium alkyl naphthalene sulfate, and0.05 part by weight of alkyl aryl sodium sulfonate dissolved in 10 partsby weight of warm water. The dispersion was then homogenized and stirreduntil the temperature of the emulsion was below about 110 F.

4 parts by weight of this isocyanate emulsion (parts in this casemeaning emulsion solids) was then added to a non-ionic vinyl acetatecopolymer latex containing 100 parts by weight of polymer. This latexwas the same as that utilized in Example I. The sheets were tested forwet strength (as in Example I), conditioned and tested as heretoforedescribed and the results appear in Table V. It clearly appears that thewet tensile of an impregnated sheet increases as the level ofimpregnation is raised.

TABLE V Sample Blocked Impregna- Wet Tensile 3 Isoeyanate 1 tion Level 21 Expressed as parts by weight per 100 parts by weight of polymer. 2Expressed as parts by weight per 100 parts by weight of paper. 3Expressed as pounds per inch.

sulfate and 0.2 part by weight of alkyl aryl sodium sulfonate dissolvedin 40 parts by weight of warm water. The dispersion was homogenized andstirred until the temperature of the emulsion was below about 110 F.

6.5 parts by weight of this isocyanate emulsion and 0.5 part by weightof a modified sodium polyacrylate were then added to an aqueous vinylacetate copolymer latex containing 100 parts by weight of polymer andthe resulting mixture was stirred until uniform. The latex is the sameas that used in Example I. The sheets were treated (as set forth inExample I), conditioned, and tested as heretofore described and theresults appear in Table VI. These results show the same general trend asthat set forth in Table V.

TABLE VI Sample Blocked Impregna- Wet Tensile 3 Isocyanate 1 tion Level2 1 Expressed as parts by weight per 100 parts by weight of polymer. 2Expressed as parts by weight per 100 parts by weight of paper. 3Expressed as pounds per inch.

In Example VI, the morpholine adduct of 4,4-methylenedi-o-tolylisocyanate was prepared, emulsified, and then dispersed in avinyl acetate copolymer latex which was later used to treat paper.

Example VII 28 grams of 4,4-methylene di-o-tolylisocyanate and grams ofmorpholine were mixed in 200 m1. of methyl ethyl ketone. The exothermicreaction which resulted was maintained at 40 C. after which a yellowishwhite precipitate was filtered off.

2 /2 parts by weight of this precipitate (which was the morpholineadduct of 4,4-methylene di-o-tolylisocyanate) was dissolved in 47 /2parts by weight of methyl Cellosolve. This solution was then added to 50parts by weight of water and the isocyanate was precipitated in the formof a fine dispersion.

7 parts by weight of this dispersion and 0.5 part by weight of amodified sodium polyacrylate (both on the dry basis) were added to avinyl acetate copolymer latex containing parts by weight polymer and theresulting mixture was stirred until uniform. The latex used was the sameas that used in Example I. The sheets were treated (as in Example I),conditioned, and tested as heretofore described and the results appearin Table VII. The same trend is shown that appears in Tables V and VIalthough a different isocyanate has been used.

TABLE VII Sample Blocked Imprcgna- Wet Tensile 3 Isoeyanate 1 tion Level2 1 Expressed as parts by weight per 100 parts by weight of polymer. 2Expressed as parts by weight per 100 parts by weight of paper. 3Expressed as pounds per inch.

The following is a list of other isocyanates which have been foundsuited for use in this procedure. The wet tensile strengths obtained onpaper which had been treated with a vinyl acetate copolymer latexcontaining one of these individual isocyanates have been found to bevery similar to the results heretofore set forth in the various tables.

Recom- Blocked Isocyanate mended Proportion 1. Bis phenyl adduct ofmethylene bis(4-phenyllsocyanate) 8. 3 2. Reaction product of phenol andtriphenyl methane triisocyanate 6. 7 3. Reaction product of morpholineand triphenyl methane triisocyanate 6. 5 4. Reaction product ofdiphenylamine and triphenyl methane triisocyanate 9.0 5. Reactionproduct of aceto-acetic ester and triphenyl methane triisocyanare 7. 96. Morphollne adduct of methylene bis(4-phenylisocyanate). 6. 4 7.Aceto-acetic ester adduct of methylene bis(4-phenylisocyanate). 7. 7 8.Diphenylamine adduct of methylene bis(4-phenylisocyanate 9.1 9. Phenoladduct of 4,4-methy1ene di-o-tolylisocyanate 7. l0. Aceto-acetic esteradduct of 4,4-methylene di-o-tolyliso cyanate. 8.4 11. Diphenylamineadduct of 4,4-methylene di-o-tolylisocy'mate 9.

1 Parts by weight per 100 parts by weight of polymer.

In many instances it is highly desirable to be able to modify theproperties of a treated sheet to meet a particular requirement. Forexample, for dynamic uses, the tensile-product of the paper is of primeimportance rather than the tensile alone. The tensile-product is theproduct of the tensile per inch width of the paper multiplied by thepercent elongation. High tensile-products may be accomplished by varyingthe ingredients in the formulation which is used to treat the paper. Tobe more specific, it has been found that the elongation and tensilestrength of a treated sheet may be varied when synthetic polymericsystems, such as butadiene-styrene and butadiene acrylonitrile areincorporated into the compound. In such instances, however, the additionpolymer must be non-ionic in nature. If, however, it is not possible touse a non-ionic system for the polymer, a system should be used whereineach of the components, in and of itself, is stable. The component whichinitially functions as a carrier for the isocyanate must be non-ionic innature, while the other component may contain a material which is ionicin nature. When mixed, these two components will form an unstable systemdue to the effect of the ionic component upon the blocked isocyanate.Before destabilization has occurred to any great extent, this fluidsystem is used to treat paper in the manner heretofore described. Normaltake up of the compound by the paper during treatment will consume thecompound before substantial destabilization takes place and furtherdestabilization will then proceed after the polymer is in place in thepaper.

A typical two component system is set up in Example VIII.

Example VIII Component A.4 parts by weight of the reaction product oftoluene 2,4diisocyanate, trimethylol propane and phenol were emulsifiedin a manner similar to that in Example I. This emulsion and 0.2 part byweight of a modified sodium polyacrylate were then dispersed in 30 partsby weight of a vinyl acetate copolymer latex. This latex was the same asthat in Example I.

Component B.0.5 part by weight of ammonium caseinate was added, withstirring, to 70 parts by weight of a 50:50 butadiene styrene syntheticrubbery polymeric latex. Subsequently 2 parts by weight of anantioxidant and 0.3 part by weight of the sodium salt of tetraaceticacid ethylenediamine was added to this liquid. The systern was mixed and1% of ammonia (based on the wet wet of the system) was added theretowith stirring.

Component A was uniformly dispersed in Component B and the resultingfluid mixture was used to treat paper in the manner heretoforedescribed.

The paper was then tested and it was found that the tensile product waseven greater than that obtained when a similar paper was treated withthe basic system heretofore described (which contained only polymers orcopolymers of vinyl acetate). To verify this increase, a series of fluidcompounds prepared according to the procedure set forth in Example VIIIwere used to treat paper. In each case, including the controls, theamount of polymeric material present in the individual compound wasvaried. Also, in the case of the control strips, the use of isocyanatewas omitted from the treatment. The amount of isocyanate used is basedupon parts by weight in a latex having 100 parts by weight of polymer.Sheets treated with these compounds were tested for tensile strength anda summary of the results is set forth in Table VIII. It appears that anincrease in wet tensile strength is obtained when a 50:50 copolymer ofbuta diene-styrene synthetic polymer is substituted for the vinylcopolymer acetate latex in the basic formulations. As is also apparent,a higher level of wet tensile may be obtained when mixtures of the vinylacetate copolymer with 50:50 butadiene styrene copolymer are used.

One of the basic advantages of the two component system is thatdestabilization of the blocked isocyanate is initiated at relatively lowtemperatures. The mixed system, in any case, however, is stable for atleast 24 hours after mixing. Therefore, the paper may be impregnatedwith the polymeric system (including the isocyanate) and destabilizationwill take place while the polymer is in place in the paper. It has beenfound that when a twocomponent system is used that a satisfactory curemay be obtained at temperatures as low as about 120 F. in as little as 3days. This is especially adaptable to paper making practices because thepaper may be treated, rolled warm, and held for several days prior toshipment.

In the following Table IX a comparison of the wet tensile is made onidentically treated samples except that one was cured at 350 F. for 2minutes, while the other was cured at 120 F. for 3 days. In any case thepaper was treated with material prepared according to Example VIII withthe exception that the amount of isocyanate used and the impregnationlevel were varied as indicated. It appears that between to of the wettensile obtained by a high temperature cure is achieved even when a lowtemperature cure is used.

TABLE IX Parts by Impregnation Wet Tensile, weight Isocy- Level, partspounds per Elongatlons anate Used by weight of square inch per 100 partsImpregnant by weight per 100 parts of Polymer by weight 350 F. F. 350 F.120 F.

of Paper As is apparent from the aforesaid data, the physical propertiesof this treated paper are improved for both dry and wet applications.The great increase in dry elongation results in paper having a very hightensile-product, a property highly desirable for dynamic uses such as inmultiwall bags. The phenomenal increase in wet strength makes the paperuseful for packaging wet products which can not otherwise be packedsuccessfully in paper bags. These improvements in the physicalproperties of paper are of extreme importance to industry. It is knownthat today over one and one-half billion pounds of paper are requiredfor the normal yearly production of multiwalltype bags. This isprimarily due to the fact that it has been found necessary to utilize asmany as 6 plies of paper in the production of these type bags. As manyas 6 plies are required in order to insure that the sidewall has thedesired degree of strength. However, by utilizing paper treated in theaforesaid manner, this poundage requirement could be substantiallyreduced. This could be done by substantially reducing the number ofplies and consequently the weight required in this type paper bag. Byutilizing paper treated in the aforesaid manner, these bags may beproduced with as little as 4 plies and, in many cases, with as little as2 plies. In many cases, even though the number of plies of such bags arereduced, the wet strength of these bags could be increased anywhere fromfour to five hundred percent. This is only one particular instance inwhich the present treatment is useful, but this is only by way ofillustration and this invention should not be limited thereto.

We claim:

1. A method of improving the physical properties of cellulose paperwhich comprises: impregnating said paper with a composition consistingessentially of a substantially non-ionic polymeric vinyl acetate latexwherein the chain portion of the vinyl acetate polymer contains hydrogenand hydroxy groups as substituents on said chain, said groups beingcapable of reacting with isocyanate groups, and an aqueous emulsion of apolyisocyanate adduct which is stable in the presence of the non-ioniclatex and non-reactive until heated to a temperature which regeneratesfree isocyanate groups in reactive form, substantially drying thetreated paper at a temperature below the temperature at which freeisocyanate groups are regenerated, and heating the dried paper to atemperature above about 140 C. for a period of time sufficient toprovide free isocyanate groups which react with the hydrogen and hydroxygroups in the paper and the polymer.

2. A method of improving the physical properties of cellulose paperwhich comprises: impregnating said paper with a composition consistingessentially of a substantially non-ionic polymeric vinyl acetate latexwherein the chain portion of the vinyl acetate polymer contains hydrogenand hydroxy groups as substituents on said chain, said groups beingcapable of reacting with isocyanate groups, and an aqueous emulsion ofbetween 2.5 and 123 parts by weight based on 100 parts by weight of saidpolymeric latex solids, of a polyisocyanate adduct selected from thegroups consisting of the phenol adducts of triphenyl methanetriisocyanate, toluene-2, 4-diisocyanate, and 4,4-methylenedi-o-tolylisocyanate; the aceto-acetic ester adducts of methylenebis(4-phenylisocyanate), triphenyl methane triisocyanate,toluene-2,4-diisocyanate, and 4,4-methylene di-o-tolylisocyanate; themorpholine adducts of triphenyl methane triisocyanate, 4,4-methylenedi-o-tolylisocyanate, and methylene bis(4-phenylisocyanate); and theadduct of toluene-2,4-diisocyanate, trimethylol propane and phenol; saidadduct being stable in the presence of the non-ionic latex andnon-reactive until heated to a temperature which regenerates freeisocyanate groups in reactive form, substantially drying the treatedpaper at a temperature below the temperature at which free isocyanategroups are regenerated, and heating the dried paper to a temperatureabove about 140 C. for a period of time sufiicient to provide freeisocyanate groups which react with the hydrogen and hydroxy groups inthe paper and the polymer.

3. The method according to claim 2 wherein the polymeric component ofthe latex is comprised of a vinyl acetate-dibutyl maleate copolymer.

4. The method according to claim 3 wherein between 2.5 and 50 parts byweight per 100 parts by weight of said copolymer latex solids of apolyisocyanate adduct are used.

5. The method according to claim 4 wherein the adduct is thediphenylamine adduct of toluene-2,4-diisocyanate.

6. The method according to claim 4 wherein the adduct is theaceto-acetic ester adduct of toluene-2,4-diisocyanate.

7. The method according to claim 4 wherein the adduct is the morpholineadduct of 4,4'-methylene di-o-tolylisocyanate.

8. The method according to claim 4 wherein the adduct is the adduct oftoluene-2,4-diisocyanate, trimethylol propane and phenol.

9. A method of improving the physical properties of cellulose paperwhich comprises: impregnating said paper with a composition comprised ofbetween 50 to 100 parts by weight of substantially non-ionic vinylacetate copolymer latex solids wherein the vinyl acetate chain portionof said copolymer contains hydrogen and hydroxy groups as substituentson said chain capable of reacting with isocyanate groups, an aqueousemulsion of about 4 parts by weight of the adduct oftoluene-2,4-diisocyanate, trimethylol propane and phenol, said adductbeing stable in the presence of the non-ionic latex and non-reactiveuntil heated to a temperature which regenerates free isocyanate groupsin reactive form, and up to 50 parts by weight of a substantiallynon-ionic butadiene-styrene copolymer latex; substantially drying thetreated paper at a temperature below the temperature at which freeisocyanate groups are regenerated; and heating the dried paper to atemperature in the range of about 120 F. to 350 F. for a period of timeranging from about 2 minutes to 3 days to provide free isocyanate groupswhich react with the hydrogen and hydroxy groups in the paper and thevinyl acetate copolymer.

References Cited by the Examiner UNITED STATES PATENTS 2,570,253 10/1951Lindquist 260-7 8.5 2,897,094 7/1961 Hayes et a1 117l55 2,994,671 8/1961Thompson 26029.7 2,994,672 8/1961 Geerdes 117155 3,001,957 9/1961 Krayet al l17155 3,005,728 10/1961 Bridgeford 8116.2 X 3,092,601 6/1963Sullivan et a1 117-155 NORMAN G. TORCHIN, Primary Examiner.

MORRIS O. WOLK, Examiner.

1. A METHOD OF IMPROVING THE PHYSICAL PROPERTIES OF CELLULOSE PAPERWHICH COMPRISES: IMPREGNATING SAID PAPER WITH A COMPOSITION CONSISTINGESSENTIALLY OF A SUBSTANTIALLY NON-IONIC POLYMERIC VINYL ACETATE LATEXWHEREIN THE CHAIN PORTION OF THE VINYL ACETATE POLYMER CONTAINS HYDROGENAND HYDROXY GROUPS AS SUBSTITUENTS ON SAID CHAIN, SAID GROUPS BEINGCAPABLE OF REACTING WITH ISOCYANATE GROUPS, AND AN AQUEOUS EMULSION OF APOLYISOCYANATE ADDUCT WHICH IS STABLE IN THE PRESENCE OF THE NON-IONICLATEX AND NON-REACTIVE UNTIL HEATED TOA TEMPERATURE WHICH REGENERATESFREE ISOCYANATE GROUPS IN REACTIVE FORM, SUBSTANTIALLY DRYING THETREATED PAPER AT A TEMPERATURE BELOW THE TEMPERATURE AT WHICH FREEISOCYANATA GROUPS ARE REGENERATED, AND HEATING THE DRIED PAPER TO ATEMPERATURE ABOVE ABOUT 140*C. FOR A PERIOD OF TIME SUFFICIENT TOPROVIDE FREE ISOCYANATE GROUPS WHICH REACT WITH THE HYDROGEN AND HYDROXYGROUPS IN THE PAPER AND THE POLYMER.
 9. A METHOD OF IMPROVING THEPHYSICAL PROPERTIES OF CELLULOSE PAPER WHICH COMPRISES: IMPREGNATINGSAID PAPER WITH A COMPOSITION COMPRISED OF BETWEEN 50 TO 100 PARTS BYWEIGHT OF SUBSTANTIALLY NON-IONIC VINYL ACETATE COPOLYMER LATEX SOLIDSWHEREIN THE VINYL ACETATE CHAIN PORTION OF SAID COPOLYMER CONTAINSHYDROGEN AND HYDROXY GROUPS AS SUBSTITUENTS ON SAID CHAIN CAPABLE OFREACTING WITH ISOCYANATE GROUPS, AN AQUEOUS EMULSION OF ABOUT 4 PARTS BYWEIGHT OF THE ADDUCT OF TOLUENE-2,4-DIISOCYANATE, TRIMETHYLOL PROPANEAND PHENOL, SAID ADDUCT BEING STABLE IN THE PRESENCE OF THE NON-IONICLATEX AND NON-REACTIVE UNTIL HEATED TO A TEMPERATURE WHICH REGENERATESFREE ISOCYANATE GROUPS IN REACTIVE FORM, AND UP TO 50 PARTS BY WEIGHT OFA SUBSTANTIALLY NON-IONIC BUTADIENE-STYRENE COPOLYMER LATEX;SUBSTANTIALLY DRYING THE TREATED PAPER AT A TEMPERATURE BELOW THETEMPERATURE AT WHICH FREE ISOCYANATE GROUPS ARE REGENERATED; AND HEATINGTHE DRIED PAPER TO A TEMPERATURE IN THE RANGE OF ABOUT 120*F. TO 350*F.FOR A PERIOD OF TIME RANGING FROM ABOUT 2 MINUTES TO 3 DAYS TO PROVIDEFREE ISOCYANATE GROUPS WHICH REACT WITH THE HYDROGEN AND HYDROXY GROUPSIN THE PAPER AND THE VINYL ACETATE COPOLYMER.